Calcium carbonate is used extensively in the paper industry as a filler component in paper. It is a low cost, high brightness filler used to increase sheet brightness and opacity. Its use has increased dramatically in the last decades due to the conversion from acid to alkaline papermaking at paper mills. Both natural and synthetic calcium carbonates are used in the paper industry. Natural carbonate, or limestone, is ground to a small particle size prior to its use in paper, while synthetic calcium carbonate is manufactured by a precipitation reaction and is therefore called precipitated calcium carbonate (PCC).
Besides its use in the papermaking industry, precipitated calcium carbonate is also used for various other purposes, e.g. as filler or pigment in the paint industries, and as functional filler for the manufacture of plastic materials, plastisols, sealing compounds, printing inks, rubber, toothpaste, cosmetics, etc.
Precipitated calcium carbonate exists in three primary crystalline forms: calcite, aragonite and vaterite, and there are many different polymorphs (crystal habits) for each of these crystalline forms. Calcite has a trigonal structure with typical crystal habits such as scalenohedral (S-PCC), rhombohedral (R-PCC), hexagonal prismatic, pinacoidal, colloidal (C-PCC), cubic, and prismatic (P-PCC). Aragonite is an orthorhombic structure with typical crystal habits of twinned hexagonal prismatic crystals, as well as diverse assortment of thin elongated prismatic, curved bladed, steep pyramidal, chisel shaped crystals, branching tree, and coral or worm-like form.
Usually, PCC is prepared by introducing CO2 into an aqueous suspension of calcium hydroxide, the so-called milk of limeCa(OH)2+CO2→CaCO3+H2O.
This process has demonstrated the capability to produce PCC with superior opacifying characteristics. PCC is typically superior in opacifying and brightening the sheet, and also as filler and/or bulk in the sheet of paper, compared to ground calcium carbonate (so-called GCC).
Alternatively, it is also known to prepare precipitated calcium carbonate by introducing an aqueous suspension of calcium hydroxide into carbonated water. Such a process can be named as an “inverse” carbonation process.
Kosin et al. describes in U.S. Pat. No. 4,888,160 an “inverse” carbonation process for the production of cuboidal calcium carbonate in a reaction vessel with a recycle piping system. The aqueous suspension of calcium hydroxide is introduced into carbonated water having a pH of 6 at a rate so that the pH of the resultant slurry formed is in a range between 8 and 9. The resultant calcium carbonate has a cuboidal shape and an average particle size of 1 to 3 μm.
Another “inverse” carbonation process for the production of cubic calcium carbonate is disclosed by Masaru et al. in JP 4031314. In this process, an aqueous calcium hydroxide slurry at a temperature of 20 to 80° C. is slowly added—at a rate of 0.001 to 0.01 fold/minute per volume of carbonated water—to carbonated water at a temperature of 20 to 60° C. until the reaction reaches a pH of 11 or less. Then the reaction product is filtered and dried. The resultant calcium carbonate has an average particle size of 1 to 10 μm.
The disadvantages of the above described “inverse” carbonation processes are the low addition rate needed to obtain the desired product as well as the low solid content of the obtained slurry.
Accordingly, there exists a need to develop an “inverse” carbonation process where the crystal habit of the resulting precipitated calcium carbonate can be governed and where a higher solid content of the precipitated calcium carbonate in the resulting slurry can be achieved. In other words, there exists a need to provide a more economical process for the production of precipitated calcium carbonate via an “inverse” carbonation process compared to the processes already known in the art.